One molecule known to initiate programmed cell death in a variety of cell types in normal and pathologic conditions is the cell-surface receptor Fas. Fas is activated by another membrane protein, the Fas-ligand (FasL). Data from our and other laboratories demonstrate that Fas expression is increased and caspase-8 is cleaved and activated after trauma both in rodent models and in human TBI specimens. We have used two endogenous molecules that inhibit Fas-mediated cell death, Bcl-xL and cFLIP-L, to develop novel means of inhibiting Fas-mediated cell death. Therefore, the hypothesis to betested is that Fasis activated by FasL after trauma and initiates programmed cell death of neurons and worsens functional outcome. The following specific aims will be addressed: Aim 1. Test whether FasL function contributes to neuronal death and behavioral dysfunction after controlled cortical impact (CCI) in vivo by testing mice with a loss-of-function mutation in FasL (gld). Determine how the loss of FasL function effects the downstream events that execute programmed cell death afterCCI. Aim 2. Test whether the Fas pathway contributes to neuronal death after CCI in vivo by overexpression of cFLIP-L, a dominant negative inhibitor of Fas-mediated cell death, using an adeno- associated viral (AAV) vector. Aim 3. Test whether overexpression of Bcl-xL in neurons by systemic infusion of Bcl-xL-TAT fusion protein prior to and after injury protects mouse brain againstCCI. Aim 4. Determine if changes in Fas,FasL, Bid and caspase-8 occur in brain and CSF samples from patients with TBI and if these changes correlate with long-term outcome. Determine if increased expression of Fas is associated with (3-amyloid (Af3) deposition. The broad long-term objective of this project is to elucidate the molecular mechanisms of neuronal cell death after trauma by studying the role of the cell-death receptor Fas and related death-regulated gene products. Thus, this project may contribute to a better understanding of not only the mechanisms of neuronal cell death in TBI, but also suggest new treatment approaches.